The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Engines may be operated in a spark ignition (SI) mode and a homogeneous charge compression ignition (HCCI) mode. The HCCI mode involves compressing a mixture of fuel and an oxidizer to a point of auto-ignition. One of the modes may be selected based on engine speed and load. In the HCCI mode, ignition occurs at several locations at a time, which makes a fuel/air mixture burn nearly simultaneously. The HCCI mode performs close to an ideal Otto cycle, provides improved operating efficiency and generates lower emission levels as compared to SI mode. However since there is no direct initiator of combustion, the ignition process tends to be more difficult to control.
To adjust operation during the HCCI mode, a control system may alter the conditions that induce combustion. For example, the control system may adjust compression ratios, induced gas temperature, induced gas pressure, or the quantity of retained or reinducted exhaust. Several approaches have been used to perform the adjustments and thus extend the HCCI operating region.
One control approach employs variable valve timing to adjust the compression ratio. For example, the compression ratio can be controlled by adjusting when intake valves close. The amount of exhaust gas retained in a combustion chamber can be controlled by valve re-opening and/or valve overlap.
During operation of the engine in HCCI mode, the exhaust gas temperatures are lower than running in a spark-injection mode. The lower exhaust temperatures in an HCCI mode may, for extended periods of time, reduce the catalyst efficiency. When the catalyst temperature drops below a predetermined temperature, the catalyst efficiency may drop below a critical threshold. When the temperature or catalyst efficiency drops below a predetermined threshold, the amount of exhaust gases emitted by the vehicle increases.